If I understand correctly, AS's agreement with ALL of the partners is that you can combine AS and the partner in an award reservation, but you can never take 2 partners in one award reservation - even if they themselves are codeshare partners.
Correct- though for the purposes of European travel, KL and AF are the same (so you could go XXX-CDG-MUC-AMS-XXX on a round trip split between both airlines).
You
can take AA to FRA and then hop a quick ICE train to MUC, though. IMO, this is probably a better option than trying to find DL award inventory, as AA usually has lots of FRA availability (though they are shutting down ORD-FRA over winter). This is probably no worse than making you or your partner (or both) go an extra leg on AS to a gateway city, so I'd consider it.
So:
DL direct via ATL
AA to FRA via DFW/ORD, train to MUC
BA direct via LHR
KL direct via AMS
AF direct via CDG